Semipermeable asymmetric cellulosic "skinned" separation membranes formed by phase inversion and solvent exchange methods are known. See U.S. Pat. Nos. 3,133,132, 3,432,585, and 3,344,214. Such membranes are characterized by a thin, dense, selectively semipermeable surface "skin" and a less dense void-containing, non-selective support region, with pore sizes ranging from large in the support region to very small proximate to the "skin." Such membranes have a serious shortcoming in that, in operation, fluxes decrease substantially over time. This decrease has been attributed to a collapse of some of the pores near the skinned surface of the membrane, resulting in an undue densification of the surface skin. One attempt at overcoming this problem has been the development of thin film composite or "TFC" membranes, comprising a thin selective skin deposited on a resilient porous support. See, for example, Riley et al., Applied Polymer Symposium No. 22, pages 255-267 (1973). While TFC membranes are less susceptible to flux decline than phase inversion-type membranes, fabrication of TFC membranes that are free from leaks is difficult, and fabrication requires multiple steps and so is generally more complex and costly.
One approach to overcoming the problem of leaks in asymmetric membranes has been the fabrication of an asymmetric gas separation membrane comprising a relatively porous and substantial void-containing selective "parent" membrane such as polysulfone or cellulose acetate that would have permselectivity were it not porous, said parent membrane being coated with a liquid such as a polysiloxane or a silicone rubber in occluding contact with the porous parent membrane, the coating filling surface pores and other imperfections comprising voids. See U.S. Pat. No. 4,230,463. However, the coatings of such coated membranes, especially where the coatings are polysiloxane, are subject to degradation by solvents inherently present in the gaseous feed streams of common acid gas separations, and otherwise tend to leach out, thus permitting either flux decline or low selectivity that is characteristic to the uncoated parent membrane.
What is needed, therefore, is an asymmetric membrane that can be inexpensively made by conventional single casting techniques, yet has a high selectivity and flux rate which do not decline in use. These needs and others are met by the present invention, which is summarized and described in detail below.